Corona generating circuits for electrophotographic printers cooperatively operating with television receivers

ABSTRACT

Corona generating pulses applied to an electrophotographic charging structure are generated in substantial time synchronism with a horizontal flyback pulse available in the television receiver and from which they are derived. The repetition rate and number of corona pulses which are applied can be selectively controlled to provide an optimum visible image on the electrophotographic recording medium. Such image is characterized by the relative absence of toner particles in the reproduced background.

[,72] Inventor:

United States Patent Bruce, Jr.

[54] CORONA GENERATING CIRCUITS FOR ELECTROPHOTOGRAPHIC PRINTERS COOPERATIVELY OPERATING WITH TELEVISION RECEIVERS Ross William Bruce, Jr., Willingboro, NJ. [73] Assignee: RCA Corporation [22] Filed: Sept. 24, 1970 [2]] Appl. No.: 75,011

[52] US. Cl ..178/6.6 A, 250/495 ZC [51] Int. Cl. ..G03g 5/02, (303g 13/04, H04n 1/30 [58] Field of Search l 78/6.6 A, 6.6 R; 250/49.5 GC,

250/49.5 ZC; 346/74 ES GE L LIiO M 15K [4 1 Apr. 11, 1972 [56] References Cited UNITED STATES PATENTS 3,496,351 2/1970 Cunningham, .lr ..250/49.5 ZC

Primary Examiner-Stanley M. Urynowicz, Jr. Assistant Examiner-Steven B. Pokotilow Attorney-Eugene M. Whitacre [5 7] ABSTRACT Corona generating pulses applied to an electrophotographic charging structure are generated in substantial time synchronism with a horizontal flyback pulse available in the television receiver and from which they are derived. The repetition rate and number of corona pulses which are applied can be selectively controlled to provide an optimum visible image on the electrophotographic recording medium. Such image is characterized by the relative absence of toner particles in the reproduced background.

7 Claims, 1 Drawing Figure Patented April 11, 1972 I N VEN TOR Ross W. Bruce, Jr. Y

Arrow/fir CORONA GENERATING CIRCUITS FOR ELECTROPI-IOTOGRAPHIC PRINTERS COOPERATIVELY OPERATING WITH TELEVISION RECEIVERS BACKGROUND OF THE INVENTION pulsed coronagenerating voltages from a television receiver in conjunction with which the electrophotographic printer operates, in particular.

2. Description of the Prior Art As is well known, in electrophotography, it is common to .apply a uniform electrostatic charge to the surface of a photoconductive layer. The charge in selected areas is then dissipated by exposing the surface to a light image to produce a resulting pattern of charges .which is rendered visible by thereafter applying finely divided developer toner particles. These particles adhere to the surface by triboelectric attraction, and enable a permanent visible image to be obtained, for example, by using thermoplastic developer particles which can be heat-fused to the photoconductive layer.

While one practice for charging the photoconductive layer employs direct current corona sources, many advantages have been found to follow from the use of high voltage corona generating pulses to expose the surface of the layer. Such an arrangement is described in US. Pat. No. 3,237,068 assigned to the same assignee as is the invention of this instant application, where it is noted that pulse charging establishes a sub stantially uniform electrostatic charge upon the electrophotographic recording medium. This is contrasted with the results obtained with direct voltage charging, where it is noted that the negative corona deposits its ions on the photoconductive layer in a non-uniform pattern, to result in non-uniform electrophotographic printing.

The use of electrophotographic printing on paper in conjunction with a television receiver is disclosed in US. Pat. No. 3,493,674, also assigned to the assignee of this invention. There, a system is described which sequentially multiplexes message representative line-scan video signals developed by an auxiliary pick-up camera with primary program video signals developed by a studio pick-up camera during predetermined portions of the vertical blanking interval thereof. More particularly, those video messages signals are inserted during a time interval corresponding to that between successive horizontal synchronizing pulses within the vertical blanking interval of each program field. The composite signal is then transmitted to the home receiver in the usual manner, where apparatus is additionally included to separate the message signals from the rest of the received signals. The separated message signals may then be recorded using a thin window type cathode-ray tube and an associated electrophotographic printer, while the primary program signals may be displayed on the kinescope of the home receiver in the conventional way. However, in those instances where direct voltages were applied to the electrophotographic charging structure to expose the surface of the photoconductive layer, it was found that a significant amount of interference was produced in the kinescope display, directly resulting from the noise associated with the random ionization of the corona source wires and picked up by the high-gain radio frequency tuner of the receiver. Furthermore, it was unexpectedly found that the high frequency, high voltage pulses which were available in the receiver for electrophotographic charging would not generally be compatible with the many types of electrophotographic printers envisioned, even though such pulse charging was noted to generate much less interference than did the direct voltage charging.

For example, it would be desirable to use a printer which would start with the beginning of a message transmission and which would stop upon its completion. One arrangement for accomplishing such operation without the need for complex paper-pulling mechanical configurations could turn-on the auxiliary receiver's high voltage power supply when printing is desired and turn-off the supply upon termination of the message reproduction. But, since that supply would generally work from the same horizontal flyback pulses of the main receiver to generate the filament voltages and electrode voltages for the thin window tube, such alternate starting and stopping of the supply would tend to upset the thin window tube potentials and, hence, its operating stability.

Besides thus requiring a pulse charging arrangement which operates separately from the supply providing the high kilovolt potentials for the thin window tube, an arrangement is needed which can be tailored for use with the particular television receiver and recording medium employed. That is, if the charging pulse is in fact generated from a horizontal flyback signal, it becomes important that no ringing pulses be present which can cause image reproduction at an unwanted time. This is of particular significance in the aforedescribed message system environment where image signals illustratively representative of stock-market information in a vertical blanking interval slot may immediately follow those signals representative of civil defense information which it is desired to reproduce. If such ringing did in fact exist, the unwanted stock market information would generally be produced due to the electrophotographic charging associated therewith, along with the desired civil defense message.

Also, it has been noted that electrophotographic recording mediums supplied by different manufacturers by and large differ in their reproduction characteristics so that, whereas a single generated pulse may be sufficient to expose the surface of one manufacturers photoconductive layer, two or more such pulses are necessary to obtain the same degree of blackness" with other manufacturers design. Thus, the need generally exists to control the number of pulses applied to the electrophotographic charging structure instead of controlling the pulse amplitude. Without such control, toner particles have also been noted to remain on the background portion of the photoconductive layer to give a pinhole or measles effect by which the paper is overcharged and a grey background appears where a white background is optimum. The need for such additional control also arises where the speed with which the paper is to be pulled past the charger is to be adjusted so that variable length messages may each be printed on the same, standard size paper.

SUMMARY OF THE INVENTION As will become clear hereinafter, the pulse charger of the invention provides high frequency, high voltage corona generating pulses which are substantially in synchronism with, or keyed by, a horizontal scanning rate pulse available in the television receiver, and controllable by a first potentiometer to provide the output at a divisible horizontal repetition rate so as to minimize the ringing problems which might otherwise arise when the printer operates with a message system of the type disclosed in US. Pat. No. 3,493,674. The pulse charger also employs a second potentiometer to provide a variable width gate keyed to an available vertical scanning rate trigger to maintain vertical resolution in the printer display and to control the number of divided horizontal repetition rate pulses coupled to the remainder of the system in accordance with paper speed and paper characteristic requirements so as to minimize the pinhole effects in the reproduced background and optimize the visible image. The divided horizontal repetition rate pulses so selected by the variable gate generator are then employed to trigger a capacitive discharge type of high voltage supply to shock-excite an inductive circuit for deriving the corona generating pulses.

A high voltage clamp is further employed to derive such pulses of one polarity only, for application to an electrophotographic charging structure having only one single wire, for example, in front of the recording medium-instead of using two sets of wires when direct voltages are employed, where the second set is in parallel array behind the recording medium. An average direct voltage component is developed in this manner, on which the pulsating potential is superimposed to supplement the pulse charging and to further decrease the marbleized density characteristic of the previous nonuniform direct voltage recording.

BRIEF DESCRIPTION OF THE DRAWING These and other advantages of the instant invention will become apparent from a consideration of the following detailed description of the single FIGURE of the drawing which represents a system according to the invention for generating high voltage, but controllable corona pulses for application to an electrophotographic charging structure.

DETAILED DESCRIPTION OF THE INVENTION Thus, referring to the drawing, pulses supplied at terminal at the horizontal synchronizing scanning rate are divided down in frequency by the stages including transistors 12, 14 and 16 in accordance with a predetermined time constant set by a first potentiometer 18. As shown, the horizontal synchronizing rate pulse is coupled by means of a resistor 20 and a capacitor 22 to the base electrode of transistor 12, the emitter electrode of which is connected to a point of reference or ground potential. Inverted pulse signals developed at the collector electrode of transistor 12 across its load resistor 24 are coupled to the base electrode of transistor 14 by means of a capacitor 26 which forms a first time constant network with a resistor 28 coupling the base electrode of transistor 14 to a negative supply voltage -V. A load resistor 30 couples the collector electrode of transistor 14 to the same +V supply as the resistor 24 couples the collector electrode of transistor 12, while a further resistor 32 couples the base electrode of transistor 16 to the collector electrode of transistor 14. The collector electrode of transistor 16 is similarly shown coupled to the +V supply by means of a load resistor 34, while the emitter electrode of transistor 16 is coupled, on the one hand, directly to ground by a lead 36 and, on the other hand, to the base electrode of transistor 16 by means of a semiconductor rectifier or diode 38 having its anode electrode coupled to the emitter and its cathode electrode coupled to the base, to which a further resistor 40 also couples the negative supply voltage -V. An additional resistor 42 is also included to couple the negative supply V to the emitter electrode of transistor 14, to which a resistor 44 and capacitor 46 are coupled, in parallel, and to ground.

Two other resistors 48 and 50 are shown, being serially coupled in the order named between the base electrode of transistor 12 and one end of the potentiometer '18, the other end of which is connected to +V supply terminal. The junction between resistors 48 and 50 is similarly coupled to the junction of a semiconductor rectifier or diode 52 and a capacitor 54 serially coupled between the collector electrode of transistor 14 and ground, with the cathode electrode of diode 52being directly connected to the transistor 14 via a lead 56. As will become clear below, potentiometer l8, resistor 50 and capacitor 54 provide a second time constant network which controls the repetition rate of pulses developed at the collector electrode of transistor 16 as a division of the input horizontal scanning frequency signal.

Also shown in the drawing for providing the variable width pulse to control the number of divided horizontal repetition rate pulses coupled to the remainder of the system are transistors 60 and 62 and a second potentiometer 64. As indicated, .vertical scanning rate pulses are coupled from input terminal 66 to the base electrode of transistor 60 via a capacitor 68 and a resistor 70. Also coupled to the base electrode of that transistor is a resistor 72 returned to the +V supply and a lead 74 connecting that electrode to the collector electrode of transistor 62. A resistor 76 further couples the collector elec trode of transistor 60 to the positive supply, while the emitter electrode of transistor 60 is directly grounded along with the corresponding emitter electrode of transistor 62. A series coupling from the collector electrode of transistor 60 to ground is lastly included as comprising a capacitor 78, the potentiometer 64 and a resistor'80 in the order named, with the junction between potentiometer 64 and resistor being further coupled to the base electrode of transistor 62.

In the operation of the illustrative system shown in the drawing, and assuming no initial charge to be stored on capacitor 54, it will be noted that transistor 12 is initially biased on", while transistors 14 and 16 are initially biased off" and on", respectively. Upon application of the leading edge of a negative going horizontal synchronizing rate pulsecoupled, for example, from the horizontal oscillator of the television receiver -the transistor 12 is switched to its oft state, causing corresponding switching to the on and off" states in the transistors 14 and 16. This switching of transistor 14 to its on condition saturates the transistor and, because of the polarities indicated, establishes substantially the same negative voltage at both the cathode and anode electrodes of the diode 52 as is provided at the emitter electrode of transistor 14. As long as this negative voltage exists across the grounded capacitor 54, the feedback provided by resistor 48 to the base electrode of transistor 12 maintains that latter transistor in its switched off condition. Such condition will 1 further be controlled by the setting of potentiometer 18 which controls the rate of positive charge coupled to capacitor 54 from the +V supply, and can be such as to insensitize transistor 12 during application of subsequently supplied horizontal scanning rate pulses to terminal 10. With the values shown in the drawing, sufficient control is present to hold such negative charge on capacitor 54 as will maintain transistor 12 in its off state for a time substantially equal to the application of five successive scanning rate pulses, to provide a frequency rate division of five to one. As the positive charge coupled to capacitor 54 builds up from potentiometer l8 and the positive +V supply, a point will eventually be reached at which the negative charge will be offset and the feedback to transistor 12 will re-establish that transistor in its initial on" Condition, readying it for the next following horizontally applied pulse.

The duration of the resulting negative going pulse developed at the collector electrode of transistor 14 and of the positive going pulse developed at the collector electrode of transistor 16 is set by the time constant network including capacitor 26 and resistor 28. The diode 38 coupling the baseemitter input of transistor 16, in this respect, limits the negative voltage swing at the base electrode of transistor 16 so as to maintain that transistor in its rated range of operation. The positive pulse developed at the collector electrode of transistor 16 will be seen to thus be developed in synchronism with, or keyed by, the horizontal input pulse.

It will also be seen that in the absence of signals, transistor 60 is initially biased on while transistor 62 is biased off. Upon the application of the leading edge of a negative going vertical scanning rate pulseapplied to terminal 66 from the vertical oscillator of the television receiver, for example, the initially conductive transistor 60 will be switched to its off" state, providing a positive going pulse at the collector electrode of that transistor. Such pulse is coupled through capacitor 78 to the base electrode of transistor 62, and at a rate determined by the setting of the second potentiometer 64, to control the eventual switching of transistor 62 to its on condition. Such switching places the collector electrode of transistor 62 at essentially the same potential as its emitter electrode, in a manner to re-establish the initial bias condition by means of which transistor 60 was held conductive. At that time, the voltage developed at the collector electrode of transistor 60 falls to its initial low level, to thus provide a positive gating pulse at the collector electrode of transistor 60 whose duration is controlledby potentiometer 64. Such pulse will also be seen to be generated in substantially time synchronism, or keyed, with the supplied vertical scanning rate pulse.

directly connected to that supply,

Also shown in the arrangement of the drawing is a switching network for a capacitive discharge supply 300, including the transistors 82 and 84 and a pair of semiconductor rectifiers or diodes 86 and 88. The cathode electrodes of these diodes 86, 88 are coupled to receive the positive going pulses just described, with the diode 86 being coupled to the collector electrode of transistor 16 and with the diode 88 being coupled to the collector electrode of transistor 60. The anode electrodes of these two diodes are coupled together and, by means of a resistor 90, are also coupled to the +V voltage supply while a capacitor 92 couples these electrodes to the base input of transistor 82. Resistors 94 and 96 respectively couple the emitter electrodes of transistors 82 and 84 to the ground potential terminal, while the collector electrode of these two transistors are respectively coupled to the positive +V voltage supply--with the collector electrode of transistor 82 being and with the corresponding electrode of transistor 84 being coupled by way of a resistor 08. A further resistor 100 couples the base electrode of transistor 84 to the emitter electrode of transistor 82, the base electrode of which is further coupled to ground by an additional resistor 102.

The switching arrangement lastly includes a silicon control rectifier (SCR) 104, a semiconductor rectifier or diode 106, a resistor 108 and a capacitor 110. As shown, both the gate electrode of the silicon control rectifier 104 and the cathode electrode of the diode 106 are grounded, while the cathode electrode of the rectifier 104 is coupled to the anode electrode of the diode 106 by means of a lead 112. The junction of these last two mentioned electrodes are further coupled, by the resistor 108 to the +V supply in one instance and by the capacitor 110 to the collector electrode of transistor 84 in the second instance. The capacitive voltage discharge supply 300 includes a transistor 120 having a collector electrode coupled by a resistor 122 to a higher, positive voltage terminal +5 and an emitter electrode coupled to ground by means of a capacitor 124. The base electrode of transistor 120 is further coupled by a resistor 126 to the +B supply and by a semiconductor diode 128 to its emitter electrode, with the cathode electrode of the diode being connected to the base electrode by a lead 130. A primary winding 132 of a transformer 134 is also shown coupled between the base electrode of transistor 120 and the anode electrode of the SCR 104, to which a series coupling including the secondary winding 136 of transformer 134 is also connected along with a capacitor 138 and a semiconductor diode 140 being returned to the reference ground potential point. The junction between the capacitor 138 and the diode 140 (i.e., the anode electrode of the diode) is coupled by means of a lead 142 to the corona charging site of the electrophotographic structure. The transformer 134 is poled for current flow in the manner shown, and may comprise a shockexcitable inductive circuit of the type generally used in photoflash equipment.

Under quiescent conditions, transistor 120 is initially biased on to establish a potential across capacitor 124 substantially equal to the +B supply, while, the silicon control rectifier 104 is initially biased off by an amount essentially equal to the forward voltage drop of the initially conductive diode 106.

With the arrangement described, it will be seen that diodes 86 and 88 and resistor 90 essentially comprise an AND circuit to couple a positive output signal to the base electrode of transistor 82 only when both signals are present at the collector electrodes of transistors 16 and 60. In response to that simultaneous occurrence, a positive going pulse corresponding in duration to the pulse developed across load resistor 34 is developed at the emitter electrode of transistor 82 and is coupled to the base electrode of transistor 84 for amplification and inversion to a negative going pulse at the collector electrode of that transistor. This negative going pulse coupled through the capacitor 1 to the cathode electrode of the SCR 104 serves to fire that device and provide a discharge path to ground for capacitor 124 by means of the primary winding 132 of transformer 134. The positive discharge is translated by the transformer polarities to a negative going pulse at the lead 142, with the diode 140 serving to clamp the positive ringing to ground. At the same time, self-commutation of the rectifier 104 occurs as the positive ring is translated back from the secondary winding 136 of the transformer 134 to the primary winding 132, to thus lower the anode electrode potential of the device 104 to a value insufficient to maintain its rated holding current; The pulse developed at lead 142 and coupled to the electrophotographic charging structure at terminal 302 (not shown) will thus be seen to be keyed with the horizontal synchronizing rate pulse supplied at terminal 10 and, since it occurs during the horizontal retrace interval for a period governed by capacitor 138 which can be made small, will not substantially interfere with the normal kinescope display of the home receiver.

By further controlling the application of any of the voltages to the system as described, it becomes possible to start and stop the charging of the recording medium separately from the applications of the high kilovolt potentials to the thin window tube and thus not affect its stability of operation. The number of pulses generated at terminal 302 is controllable by the duration of the vertically generated gate pulse at transistor 60 under control of the potentiometer 64 while the frequency at which the pulses recur is established by the setting of the control potentiometer 18. With the arrangement having the values shown in the drawing, and with the type classification for the transistors and such items following, the pulse generated at terminal 302 is a negative going pulse of some 10 kv. amplitude, superimposed upon an average d-c component of minus 2 kv. This combined d-c voltage with a superimposed pulsating potential has been noted to produce such substantially uniform electrostatic charging upon the electrophotographic recording medium as to obtain a black printing on a white background with little pinhole or marbleized density recording as had been previously found.

Furthermore, it was found that such substantial uniformity of charging existed for a wide range of recording medium characteristics and for different paper speeds, which theretofore drastically affected the background of the reproduced record. The control afforded by the potentiometer 64 in this case was of most significance in regulating the exposure time for the photoconductive surface layers while the control afforded by the potentiometer 18 was of most significance in controlling the generation of electrophotographic ringing charge pulses in those systems where differing message informations were representable by signals occupying adjacent horizontal line intervals. One such system is that described in the aforementioned US. Pat. No. 3,493,673 where auxiliary information was transmitted during the vertical blanking interval, but such ringing problem will be seen to equally exist in those systems where alternate messages are sent during adjacent active line-scanning times. Either or both of the potentiometers 18 and 64 can be provided as internal controls in an electrophotographic printer to be initially set up in the establishment of a system utilizing a predetermined type of recording medium, or can alternatively be provided for home owner control to enable comparable recording quality to be obtained from electrophotographic paper of varying characteristics and in system arrangements of differing paper speeds to match those speedsdictated at the transmitting terminal.

Typical of such type classifications for those components are identified as such in the drawing are as follows:

Transistor l2 2N3643 Transistor 14 2N3643 Transistor 16 2N3643 Transistor 60 2N3643 Transistor 62 2N3638 Transistor 82 2N3643 Transistor 84 2N3440 Transistor 2N3440 Diode 38 1N34 Diode 52 IN34 Diode 86 lN34 Diode 88 lN34 Diode 106 lN3254 Diode 12B IN3254 Diode 140 50 RCA 30 SCR 104 2N3529 Transformer I34 AMGLO MT-55 While there has been described what is considered to be a preferred embodiment of the pulse charger of the present invention, it will be readily appreciated by those skilled in the art that modifications may be made without departing from the scope ofthe teachings herein. Thus, for example, the system can be modified by grounding the cathode electrode of the silicon control rectifier 104 and by applying a slight negative voltage to its control electrode to initially bias off the device. A positive going pulse obtainable from a point such as the emitter electrode of transistor 82 can then be capacitively coupled to the gate electrode of the rectifier 104 to control the switching operation in a manner analogous to that described.

What is claimed is:

l. A pulse generating circuit for electrostatically charging an electrophotographic recording medium comprising:

an inductive shock-excitation circuit for generating high voltage pulses;

first means for supplying keying pulses of a first frequency;

second means for supplying keying pulses of a second frequency;

control means coupled to said first and second means for multiplexing said first and second frequency keying pulses to provide high voltage pulses of a third frequency that is lower than said first frequency but higher than said second frequency for application to said inductive circuit to shock-excite said high voltage pulses in consecutive predetermined numbers in accordance with reproductive characteristics of the recording medium and with the speed with which said medium is caused to pass coronagenerating elements of said electrostatic charging system; and

rectifier and capacitive means connected in series with each other and to said inductive shock-excitation circuit for clamping said high voltage pulses to provide signal excursions substantially of only one polarity to said corona generating elements.

2. The pulse generating circuit of claim 1 wherein said first means supplies keying pulses having a frequency equal to the horizontal scanning rate of a television receiver and wherein said second means supplies keying pulses having a frequency equal to the vertical scanning rate of said receiver.

3. The pulse generating circuit of claim 2 wherein said control means generates high voltage pulses having a frequency equal to a sub-multiple of the horizontal scanning rate and which are synchronized substantially with the leading edges of said first and second frequency keying pulses.

4. The pulse generating circuit of claim 3 wherein said control means includes first amplifier stage initially biased to a first conductive condition and switched to a second condition in response to the leading edge of a first keying pulse supplied by said first means and wherein a feedback network including a first time constant circuit is included to control the transition of said amplifier stage back to its said first condition so as to be responsive to subsequently supplied keying pulses of said first frequency.

5. The pulse generating circuit of claim 4 wherein said control means also includes a second amplifier stage initially biased to a first conductive condition and switched to a second condition in response to the leading edge of a first keying pulse supplied by said second means and wherein a feedback network including a second time constant circuit is included to control the transition of said amplifier stage back to its said first condition so as to be responsive to subsequently supplied keying pulses of said second frequency.

6. The pulse generating circuit of claim 5 wherein said first time constant circuit is coupled to an output electrode of said first amplifier stage to selectively control the transition of said amplifier stage and to regulate the fre uency of pulses developed thereby to a sub-multiple of the requency of said first supplied keying pulses.

7. The pulse generating circuit of claim 6 wherein said second time constant network is coupled to an output electrode of said second amplifier stage to selectively control the transition of said amplifier stage and to regulate the duration of pulses developed thereby for multiplexing with said submultiple frequency pulses.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 655, 91 Dated April 11, 1972 Invent0r($) Ross William Bruce, Jr;

It is certified that error appears in the above-identified patent and thateaid Letters Patent are hereby corrected as shown below:

Column 6, Line 64 after "components" delete "are" and insert--- not Signed and sealed thist5th day of December 1972.

(SEAL) Attest:

EDWARD M-FLETCHERJR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM 0-1050 (10-59) -gc Q375. 5g

UVS. GOVERNMENT PRINNNG OFFICE I 9.9 0-16-334 3530 elm UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 9112 Dated April 11, 1972 ln fl Ross William Bruce, .Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, Line 64 after "components" delete "are" and insert not Signed and 'sealed this 15th day of December 197-2.

(SEAL) Attest:

EDWARD M.FLETGHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 w u.sv GOVERNMENY PRINYING OFFICE: 19.9 0-366-834 FORM PO-IOSO (10-69) 1 353 0 elm 

1. A pulse generating circuit for electrostatically charging an electrophotographic recording medium comprising: an inductive shock-excitation circuit for generating high voltage pulses; first means for supplying keying pulses of a first frequency; second means for supplying keying pulses of a second frequency; control means coupled to said first and second means for multiplexing said first and second frequency keying pulses to provide high voltage pulses of a third frequency that is lower thAn said first frequency but higher than said second frequency for application to said inductive circuit to shock-excite said high voltage pulses in consecutive predetermined numbers in accordance with reproductive characteristics of the recording medium and with the speed with which said medium is caused to pass corona-generating elements of said electrostatic charging system; and rectifier and capacitive means connected in series with each other and to said inductive shock-excitation circuit for clamping said high voltage pulses to provide signal excursions substantially of only one polarity to said corona generating elements.
 2. The pulse generating circuit of claim 1 wherein said first means supplies keying pulses having a frequency equal to the horizontal scanning rate of a television receiver and wherein said second means supplies keying pulses having a frequency equal to the vertical scanning rate of said receiver.
 3. The pulse generating circuit of claim 2 wherein said control means generates high voltage pulses having a frequency equal to a sub-multiple of the horizontal scanning rate and which are synchronized substantially with the leading edges of said first and second frequency keying pulses.
 4. The pulse generating circuit of claim 3 wherein said control means includes first amplifier stage initially biased to a first conductive condition and switched to a second condition in response to the leading edge of a first keying pulse supplied by said first means and wherein a feedback network including a first time constant circuit is included to control the transition of said amplifier stage back to its said first condition so as to be responsive to subsequently supplied keying pulses of said first frequency.
 5. The pulse generating circuit of claim 4 wherein said control means also includes a second amplifier stage initially biased to a first conductive condition and switched to a second condition in response to the leading edge of a first keying pulse supplied by said second means and wherein a feedback network including a second time constant circuit is included to control the transition of said amplifier stage back to its said first condition so as to be responsive to subsequently supplied keying pulses of said second frequency.
 6. The pulse generating circuit of claim 5 wherein said first time constant circuit is coupled to an output electrode of said first amplifier stage to selectively control the transition of said amplifier stage and to regulate the frequency of pulses developed thereby to a sub-multiple of the frequency of said first supplied keying pulses.
 7. The pulse generating circuit of claim 6 wherein said second time constant network is coupled to an output electrode of said second amplifier stage to selectively control the transition of said amplifier stage and to regulate the duration of pulses developed thereby for multiplexing with said sub-multiple frequency pulses. 